stash: Don't fail if work dir contains file named 'HEAD'

stash: Don't fail if work dir contains file named 'HEAD'

When performing a plain "git stash" (without --patch), git-diff would fail
with "fatal: ambiguous argument 'HEAD': both revision and filename". The
output was piped into git-update-index, masking the failed exit status.
The output is now sent to a temporary file (which is cleaned up by
existing code), and the exit status is checked. The "HEAD" arg to the
git-diff invocation has been disambiguated too, of course.

In patch mode, "git stash -p" would fail harmlessly, leaving the working
dir untouched. Interactive adding is fine, but the resulting tree was
diffed with an ambiguous 'HEAD' argument.

Use >foo (no space) when redirecting output.

In t3904, checks and operations on each file are in the order they'll
appear when interactively staging.

In t3905, fix a bug in "stash save --include-untracked -q is quiet": The
redirected stdout file was considered untracked, and so was removed from
the working directory. Use test path helper functions where appropriate.

Signed-off-by: Jonathon Mah <me@JonathonMah.com>
Acked-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>